To brown the butter, place 1 stick of butter in a medium saucepan on medium high. Melt butter, stirring only occasionally. Once melted, begin stirring often. Butter will crackle and pop as it browns and it will start to foam slightly.

Once sizzling sound stops, continue stirring often until golden brown bits begin to collect on the bottom of the pan. The butter will smell slightly nutty and aromatic.

Remove from heat, pour into the bowl of a stand mixer fitted with a paddle attachment and allow to cool for 1-2 minutes before adding the other 4 TBSP of butter. Use a spatula to stir until the remaining butter is melted.

Add the brown sugar and granulated sugar to the bowl and mix on medium speed for 2 minutes. Scrape down sides of the bowl.

With the mixer on low speed, add the egg and egg yolk, incorporating the egg fully before adding the yolk. Add the vanilla and mix on low speed to combine.

In a separate bowl, whisk together flours, baking soda and salt. Add the dry ingredients to the butter and sugar mixture in two additions. Mix only until a few streaks of flour remain. Do not over mix.

Remove the bowl from the stand mixer and stir in the chopped chocolate and toffee pieces by hand using a wooden spoon or spatula.

Use a 2 TBSP cookie scoop to scoop dough onto a parchment lined baking sheet. Cover with plastic wrap and chill overnight.

Preheat oven to 350 degrees. Place cookie dough 2 inches apart on parchment lined baking sheets. Bake for 10 minutes, rotating the pan halfway through the baking cycle. Cookies are done when the edges are set and the center of the cookies is barely cooked through. Do not over bake. Cookies will continue to bake as they cool on the pan.

Remove the pan from the oven and let cookies rest for 5-10 minutes before transferring them to a cooling rack. Best served warm. Sprinkle with flakey sea salt if desired.